International audienceThe MAVEN spacecraft launched in November 2013, arrived at Mars in September 2014, and completed commissioning and began its one-Earth-year primary science mission in November 2014. The orbiter’s science objectives are to explore the interactions of the Sun and the solar wind with the Mars magnetosphere and upper atmosphere, to determine the structure of the upper atmosphere and ionosphere and the processes controlling it, to determine the escape rates from the upper atmosphere to space at the present epoch, and to measure properties that allow us to extrapolate these escape rates into the past to determine the total loss of atmospheric gas to space through time. These results will allow us to determine the importance of loss to space in changing the Mars climate and atmosphere through time, thereby providing important boundary conditions on the history of the habitability of Mars. The MAVEN spacecraft contains eight science instruments (with nine sensors) that measure the energy and particle input from the Sun into the Mars upper atmosphere, the response of the upper atmosphere to that input, and the resulting escape of gas to space. In addition, it contains an Electra relay that will allow it to relay commands and data between spacecraft on the surface and Earth

Abstract. We report observations of "fast solitary waves" that are ubiquitous in downward current regions of the mid-altitude auroral zone. The single-period structures have large amplitudes (up to 2.5 V/m), travel much faster than the ion acoustic speed, carry substantial potentials (up to ~100 Volts), and are associated with strong modulations of energetic electron fluxes. The amplitude and speed of the structures distinguishes them from ion-acoustic solitary waves or weak double layers. The electromagnetic signature appears to be that of an positive charge (electron hole) traveling anti-earthward. We present evidence that the structures are in or near regions of magnetic-field-aligned electric fields and propose that these nonlinear structures play a key role in supporting parallel electric fields in the downward current region of the auroral zone.

The contribution to the total energy flux at these altitudes from Poynting flux associated with Alfven waves is comparable to or larger than the contribution from the particle energy flux and 1-2 orders of magnitude larger than that estimated from the large-scale steady state convection electric field and field-aligned current system.

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Funnel‐shaped, low‐frequency radiation, as observed in frequency time spectrograms, are frequently found at the Earth's magnetic equator. At the equator the radiation often extends from the proton cyclotron frequency up to the lower hybrid frequency. Ray‐tracing calculations can qualitatively reproduce the observed frequency‐time characteristics of these emissions if the waves are propagating in the fast magnetosonic mode starting with wave normal angles of ∼88° at the magnetic equator. The funnel‐shaped emissions are consistent with generation by protons with a ring‐type velocity space distribution. A ring‐shaped region of positive slope in the velocity space density distribution of protons is observed near the Alfvén velocity, indicating that the ring protons strongly interact with the waves. Ray‐tracing calculations show that for similar equatorial wave normal angles lower‐frequency fast magnetosonic waves are more closely confined to the magnetic equator than higher‐frequency fast magnetosonic waves. For waves refracted back toward the equator at similar magnetic latitudes, the lower‐frequency waves experience stronger damping in the vicinity of the equator than higher‐frequency waves. Also, wave growth is restricted to higher frequencies at larger magnetic latitudes. Wave damping at the equator and wave growth off the equator favors equatorial wave normal angle distributions which lead to the funnel‐shaped frequency time characteristic.

Coupling between the lower and upper atmosphere, combined with loss of gas from the upper atmosphere to space, likely contributed to the thin, cold, dry atmosphere of modern Mars. To help understand ongoing ion loss to space, the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made comprehensive measurements of the Mars upper atmosphere, ionosphere, and interactions with the Sun and solar wind during an interplanetary coronal mass ejection impact in March 2015. Responses include changes in the bow shock and magnetosheath, formation of widespread diffuse aurora, and enhancement of pick-up ions. Observations and models both show an enhancement in escape rate of ions to space during the event. Ion loss during solar events early in Mars history may have been a major contributor to the long-term evolution of the Mars atmosphere.

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The theta aurora is a remarkable configuration of auroral and polar cap luminosities for which a generally sun‐aligned transpolar arc extends contiguously from the dayside to nightside sectors of the auroral oval. Four individual occurrences of theta aurora over earth's northern hemisphere are examined in detail with the global auroral imaging instrumentation on board the high‐altitude, polar‐orbiting spacecraft DE 1. Simultaneous measurements of fields and plasmas with this high‐altitude spacecraft and its low‐altitude, polar‐orbiting companion, DE 2, are examined in order to establish an overview of auroral and polar cap phenomena associated with the appearance of the theta aurora. For these series of observations, two general states of the polar cap are found corresponding to (1) a bright, well‐developed transpolar arc and (2) a dim or absent transpolar arc. During periods of a relatively bright transpolar arc the plasma convection in the polar cap region associated with the transpolar arc is sunward. Elsewhere over the polar cap the convection is antisunward. The convection pattern over the auroral zones and polar cap is suggestive of the existence of four cells of plasma convection. Field‐aligned electron acceleration into the polar atmosphere and field‐aligned current sheets are present in the transpolar arc plasmas. This electron precipitation and these current sheets are relatively absent over the rest of the polar cap region. The transpolar arc plasmas exhibit similar densities and ion compositions relative to those plasmas observed simultaneously over the poleward zone of the auroral oval. The ion compositions include hot H+, He++, and O+ ions and thus are of both ionospheric and solar wind origins. Principal hot ions in the remainder of the polar cap region are H+ and He++, indicating access from the magnetosheath for these ions. Low‐energy electrons identified with a magnetosheath source are also present in this region. The dominant thermal ions in the polar cap region are O+ ions flowing upward from the ionosphere. These thermal ions are heated along magnetic flux tubes within the transpolar arc plasmas. Pairs of current sheets with oppositely directed current densities occur in the transpolar arc region and with magnitudes similar to those associated with the poleward zones of the auroral oval. The upward currents are carried by electrons accelerated by a field‐aligned potential. Funnel‐shaped auroral hiss and broadband electrostatic noise are associated with the presence of the transpolar arc plasmas. Energetic solar electrons are employed to show that the magnetic field lines threading both the transpolar arc and the poleward zone of the auroral oval are probably closed. In contrast, the accessibility of these electrons to the remainder of the polar cap indicates that these polar regions are characterized by a magnetic topology that is connected directly to field lines within the interplanetary medium. Thus the overall character of the transpolar arc region appears to be very similar to that obser...